1. Field of the Invention
The invention relates to a motion and power transfer arrangement. More particularly, the invention pertains to an improvement in swashplate design.
2. Description of Related Art
Swashplates are well known and have been used for many years in such applications as hydraulic pump motors. They are used to transfer motion between a rotating shaft and one or more translating components reciprocating on axes parallel to the shaft and disposed around it. The conventional design often has many sliding, rotating and translating parts, mechanical efficiency may be low and wear rates may be high.
A basic swashplate typically consists of a disc mounted at an inclined angle on a shaft. Rotating the shaft will cause the edges of the disc to precess in a direction parallel to the shaft axis in simple harmonic motion compared to a fixed point in space. Mechanical connection may be made by a variety of means to interact with pistons, shuttles or other components, which are required to reciprocate. For these components to interact with the swashplate and transfer forces one way or the other, three relative motions must be accommodated by sliding and/or rotating mechanical connections.
The first of these is the rotation of the disc itself which may be accommodated by slipper plates sliding on the face of the disc. Alternatively, the disc may be mounted on a skewed bearing so that it stays rotationally stationary whilst precessing about its center point. The precessive motion is similar to the final motion of a coin spun on a flat surface just before it lies flat and stops.
The second factor is the arcuate motion of a point on the rim of the disc. As the disc precesses, any point on the rim will describe part of a spherical surface. In the case of the alternative above, where the disc does not rotate, a reference point will describe an arc about an axis normal to the shaft axis. Thus its radial distance from the shaft axis will vary. If we intend to connect this motion to a part sliding parallel to the shaft axis and at a constant distance, we must accommodate the change in distance between the disc edge all the sliding axis due to this arcuate motion of the rim of the disc.
The third motion is the xe2x80x98skewxe2x80x9d angle between the plane of the disc and the axis of the reciprocating part due to the inclination of the disc. As the disc precesses, the orientation of the angle will change. At one extreme of reciprocating movement the radial angle is greatest whilst the tangential angle is normal to the sliding axis. At the mid-stroke position, the radial angle is normal and the tangential angle is at a maximum corresponding to the disc inclination.
To summarize, in order to transfer drive from the disc to the parallel reciprocating part, one must accommodate disc rotation, compensate for arcuate motion and arrange for the connection to allow angular displacement in three axes simultaneously. The more efficient existing solutions are complex and expensive to manufacture. Lower cost ones utilize crude, sliding frictional contact, which has severe penalties in wear and mechanical efficiency.
It is the aim of the present invention to eliminate the problems associated with current swashplate designs. The present invention provides a mechanism acting as a swashplate, which has few separate parts and is easy to manufacture.
The present invention transmits power and motion between the parts by rolling and meshing contact between the parts. This provides low wear and mechanical efficiency. The latter attribute is especially valuable when used in variable stroke hydraulic motors, where it will dramatically improve the mechanical efficiency at low swashplate inclination angles. A mechanism according to the present invention is very simple mechanically, needing a minimum number of easily mass-produced parts.
The arrangement of the present invention is very compact compared to existing designs and occupies a minimal swept void volume to accommodate it. This is an advantage over existing solutions in reducing the overall size and weight of machines using this invention.
Accordingly, in one embodiment of the invention, there is provided a rolling element swashplate comprising a part-spherical toothed swash-ball and one or more cylindrical racks meshing with the teeth on the ball. The ball is carried on a main shaft by a well-known rolling element bearing, which is mounted at an inclination to the shaft axis by an angled boss. This angle may be fixed or adjustable, for instance to vary piston stroke in an hydraulic machine. Rotation of the shaft will cause the equatorial plane of the ball to precess as described above, whatever the relative rotational position of the ball about its bearing.
The outer surface of the ball resembles a globe with the polar areas removed. This surface is covered with ridges and grooves that run around the ball parallel to the equatorial plane and form the teeth. The geometric form of the surface can be described as a solid of revolution derived by rotating a gearwheel with conventional or modified involute teeth about an axis across its diameter.
Viewed in cross-section the ball rocks about its center as the shaft is rotated, from maximum inclination one way to the other extreme and back. Motion of the teeth thus resembles a spur gear oscillating about its center and a rotational mid-point.
The reciprocating part slides along a fixed axis parallel to the shaft axis and carries a cylindrical rack rotatable about the part""s axis. The cylindrical rack has teeth running round it normal to the axis, that match those on the ball and mesh with them. As the main shaft turns, the point on the ball nearest to the parallel part xe2x80x9crocksxe2x80x9d in section as described above, the mesh of the teeth driving the part in simple harmonic motion.
When the ball is in the mid-position and the rack and part are at mid-stroke, the tangential skew angle will be at a maximum and equal to the xe2x80x9cdisc inclinationxe2x80x9d or xe2x80x9cswash anglexe2x80x9d. In order to accommodate this skew, the profile of the teeth on the rack is modified progressively along its length to allow clearance for the teeth to mesh obliquely as the rack passes the mid-point.